Electroplating with zinc



Patented Apr. 10, 1923,

- UNITED STATES PVATENT OFFICE.

JOSEPH HAAS, JR., MUNCIE, INDIANA, ASSIGNOR TO THE ROESSLER G: HASSLACEER CHEMICAL COMPANY, OF NEW YORK,

N. Y.,'A CORPORATION or NEW YORK.

EIiECTROPL A TING- WITH ZINC.

li o Drawing. Application filed September T 0 all wlwm it my concem,

- Be it known that I, JOSEPH HAAS, Jr., citizen of the United States, and resident of Muncie, in the county of Delaware and State 5 of Indiana, have invented certain new and useful Improvements in Electroplating with Zinc, of which the following is; a spec1fi cation.

My invention. relates in general to the electroplating of zinc or its alloys upon iron orsteel in cyanide baths. Such cyanide plating baths are widely used because of their greater throwing power as compared with other baths which have been proposed for zinc plating. I

Hitherto the commercial zinc plating of ferrous articles in cyanide baths has been subject to certain limitations. For example, it has been considered impracticable in cyanide baths to zinc plate iron or steel castings at all. Morover, the plating of articles of varoughtiron or wrought steel has been attended with some difficulty in respect to articles bf irregular surface, the difiiculty being that those portions of the article which aremore remote from the anode receive too thin a coating. In the plating of small'ferrous articles this difiiculty becomes 'so pronounced that hitherto it has been deemed impracticable to zinc plate such articles by the basket method in cyanide baths.

In investigations to determine the cause of these difficulties, I have found that it is due to the relatively low discharge potential of hydrogen upon cast iron, which causes hydrogen to be deposited rather than zinc. Researches also show that the single potential of an alloy is lower than that of its controlling constituent in the electrochemical series, andwith certain exceptions, that the lower the electrode potential of a metal or alloy, the more easily is hydrogen deposited upon it. It has also been observed in plating cast iron and steel in the same bath, that more hydrogen is evolved upon the cast iron than on the steel, evidencing that the discharge potential of hydrogen on cast iron is much lower than that of hydrogen on steel. This is furtherevidenced by the cominon practice in plating copper on cast iron to use less free cyanid than plating on steel, too much cyanid n either case causing only evolution of hydrogen.

16, 1922. Serial N0. 588,567.

In order to successfully deposit zinc on cast iron in a cyanid bath, the factors to be considered are the discharge potential of hydrogen; the electrode potential of the cathode metal; and the electrode potential of the metal to be deposited, all three being affected by the crystalline structure of the cast iron cathode. The general rule may be said to be that if the discharge potential of hydrogen on the cathode metal is low, or below the electrode potentialof the deposited metal, and the difi'erencebetween the electrode potential of the deposited metal and the discharge potential of hydrogen is great, hydrogen only is deposited. If the difference is not great, *both hydrogen and metal are depositedwhile if the discharge potential. of hydrogen considerably exceeds'the electrode potential of the deposited metal, the metal alone is deposited.

\Vhere cast iron is first plated ,with copper or'nickel, zinc can readily be deposited from a cyanid bath, due to the discharge poten-' I tial of hydrogen being raised, but this is an added expense and coatings of copperor nickel on cast iron are likely tohbe porous, and when porous arenot corrosion-preventative but stimulate corrosion.

The problem is solved according to -this invention by bringing the discharge potential of hydrogen closer to the potential of zinc in a cyanid bath without an extra electroplating bath, by depositing the zinc in the presence of ametal electrochemically. active to prevent corrosion, such .a 'metal is mercury which has the effect in a cyanid electroplating bath of bringing the discharge potential of hydrogen so near the potential of zinc that zinc will be deposited 4 together with a small percentage of mercury. In other words, the presence of mercury has the effect of making the difference zinc on cast articles not heretofore satisfactorily coated with zinc by electroplating.

Other improvements'result from my invention among which are the following:

1. The'color, smoothness, fineness of grain structure. resistance to corrosion, and hardness of the zinc mercury deposit are superior" to those formerly obtained of zinc alone fromv cyanid baths.

2. The current efiiciency at the cathode is markedly increased.

'3. The anode wastage due to chemical corrosion is decreased.

4. The zinc coatings do not readily stain in the solution or when dried out, or when subsequently handled during assembling.

As illustrations of my invention, I will now describe the same as applied to the zinc plating of various types ot'iron and steel articles.

Example I: to plate gray or white iron castings such as pipe fittings, for example, I proceed as follows:

The fitting is first cleaned, preferably by sand blasting, until all of the scale and loose dirt are removed, and a clean iron surface is exposed. Should the surface of the fitting again become'rusted, or otherwise fouled and dirt before going tothe platin tank, it should then be agaln cleaned pre erably by dipping in muriatic acid solution of about 1.09 specific gravity after which it is rinsed in clean water and allowed to remain in a hot caustic solution containing one to five per cent NaOI-I until ready for the plating operation.

The fitting so cleaned may then be dipped in a mercury solution containing for example;

Mercuric chloride 4 oz. Nitric acid sp. g. 1.4"- oz. Water to make gallon.

The fitting is then immediately suspended in a zinc cyanide plating solution to which of' mercury must not be permitted to become excessive inasmuch as in that event the permanence of the coating will be seriously impaired.

ExampleII: To plate large articles of wrought iron or wrought steel, such as automobile rims, the article instead of being sand blasted is cleaned by pickling alternately in alkaline and acid cleaning solutions until a chemically clean metallic surface is exposed. It maythen be dipped in the mercury solution and plated as described in Example I. When plated, the rim is rinsed in cold and hot water, and air dried. The deposits thus obtained are tough and of pleasing appearance and are more resistant to corrosion and abrasion than any hitherto obtained in the art.

Example III: To plate burrs, bolts, screws, and other small articles of iron or steel, I proceed as in Example II, except that the articles are handled throughput in coarse-meshed wire baskets and are plated with a current of full six volts. The deposits obtained are of excellent color and durability; moreover, the plate is of uniform thickness throughout all parts of the basket.

As I believe I am the first to discover a means for increasing the potential of the cathode in zinc cyanide plating, and thereby reducing evolution of hydrogen and increasing the amount and quality of the coating, besides producing a novel coating I do not limit myself to any specific way of introducing the mercury except as specifically re uired by the appended claims.

ithin the broad scope of the invention,

has been added a small amount of a soluble-. v btain d similar results by dipping mercury compound. Excellent results are obtained from a bath of thefollowing com position:

Water 1 gal. Zinc cyanide 4 ozs. Sodium cyanide 4 ozs. Sodium hydroxid s 2 ozs. Mercuric oxid oz.

This may be accomplished by immersing the cleaned zinc anode in a saturated solution of mercuric chloride for a suitable period, forty-eight hours, for example. During the immersion the surface is scoured from time .to time with a wire brush or other suitable means.

The solution is maintained at a temperature of from 40 to 50 C. The preferred voltage is from three to six volts depending 1651(1)!) the quantity of work, being plated.

and then' air. dried. The bath must be replenished from time to time as the several ingredients become depleted but the amount the cathode first in a,solution of a mercury salt and then plating in an ordinary cyanid bath without added mercury salt; and by plating in a cyanid bath with added mercury salt wlthout dipping. In the former case, the mercury becomes alloyed with the first zinc deposited but upon continued plating, the rcentage of mercury diminishes, and final y the coating may be practically pure zinc. With mercur in the bath, the coating is uniformly a loyed with from 1.5% 3

to 2.0% of mercury. The most resistant and satisfactolg coatings have about 2.0% of u mercury. f the percentage is below about 0.2%, the advantages resulting from its use are small and its use is limited. If the percentage of mercury is above about 5.0%, the coatings are unsatisfactory in texture, are brittle and of little utility. I do not limit myself to the aforesaid mercury compounds; as I may use any mercuric compounds which is soluble in the bath. Among such compounds the oxide is preferred for the reason that it introduces into the bath no foreign anion. \Mercurous compounds are not recommended as they are in part reduced b the bath with the precipitation of metal ic mercury. In the preliminary dipsolution I may use mercuric nitrate, sulphate, etc.

I claim:

1. In zinc electroplating from a cyanid bath, the step which consists in depositing loyed with a smal 3. Inplatiug ferrous articles with zinc in ticles which consists in in an acid solution of mercuric chlorlde and zinc in the. fpresence of mercury to produce a coating o zinc alloyed with a small percentage of mercury.

2. In zinc electroplating from a cyanid bath, the step which consists in adding to a cyanid bath to be electrolyzed, a mercury salt capable of resisting reduction by the bath with consequent precipitation of metallic mercury to lproduce a zinc coating alpercentage of mercury.

c anide baths, the ilu n'ovcment which con- -s1s ts in depositing mercury in conjunction with the zinc. I

4. The method of zinc plating ferrous articles which comprises applying to the surface of the article a liquid medium containing mercury and plating the article in a zinc bath containing cyanide.

5. The method of zinc plating ferrous articles which comprises coating the article with mercury and platin 'gthe article in a zinc bath containing cyanide and a soluble, mercury compound. I

' G. The method of zinc plating ferrous articles which comprises applying to the surface'of the article a liquid medium contain-j ing mercury and plating the article in a zinc bath containing cyanide anda'sol-uble. mercury com ound. i

. 7 The me 10d of zinc plating ferrous articles which comprises dipping the article in a solution of a mercury compound and platin the article in a zinc hath containing cyani eand a soluble mercury compound.

8. The method of zinclatmg ferrous aripping the article ticles which comprises oxide per gallon.

Lin a zinc cyanide amount of soluble mercury compound.

plat-in the article in a'zinc bath containingcyani e and a soluble mercur compound.

9. The process of zinc latlng ferrous ar- I .ipping the article m a solution containing approximately Mercuric chloride 4 oz. Nitricacidconcentra,ted oz.

Water to make 1 gallon.

and then plating the articlein a bath of approximately the following composition:

\Vater 1 gallon. Zinc cyanide 4 oz.

Sodium cyanide 4 oz.

boduun hydromde 2 oz.

Mercuric oxuL; oz.

10. The method of zinc plating ferrous artlcleswhich comprises coating the article with mercury and then plating the article 13. The method of zinc plating ferrous articles which comprises plating the article ath containing a small 14. The method of zinc electroplating ferrous articles whichconsists in plating the article in a zinc bath containing cyanide and approximately oz. of mercury per gallon in soluble form.

Signed at'Muncie in the county of Delaware and State of Indiana this 2nd day of September A. D. 1922.

JOSEPH HAiAs, Ja. 

